1. Field of the Invention
The present invention relates to a transfer device which transfers a material 1 such as toner images. In addition, the present invention also relates to an image forming apparatus including a transfer device, and a method for evaluating an electrical property of a combination of two or more members.
2. Discussion of the Background
Image forming apparatuses in which a toner image formed on an image bearing member such as photoreceptors is primarily transferred to an intermediate transfer medium and the toner image is secondarily transferred to a receiving material are well known. These image forming apparatuses typically include a primary transfer roller which serves as a bias application roller configured to apply a primary bias. Such a primary transfer roller and the image bearing member sandwich the intermediate transfer medium to form a primary nip at which the primary image transfer is performed. Further, one of the stretching rollers which tightly stretch the intermediate transfer medium, and an opposing rollers and which the intermediate transfer medium to form a secondary transfer nip at which the secondary image transfer is performed. In this regard, a secondary bias is applied to one of the stretching roller and the opposing roller so that the toner image is well transferred to the receiving material. The roller to which a secondary bias is applied is hereinafter referred to as a secondary transfer roller.
Ion-conducting materials are used for primary and secondary transfer rollers because the unevenness of resistance of the rollers can be reduced. In addition, ion-conducting materials are also used for intermediate transfer media.
However, transfer rollers and intermediate transfer media including an ion-conducting material have a drawback in that the electric resistance thereof increases after repeated use due to application of a transfer bias thereto (this problem is hereinafter referred to as an electric resistance increasing problem). A transfer roller or an intermediate transfer medium having an increased electric resistance decreases the electric field at the primary or secondary transfer nip, resulting in occurrence of a transfer problem in that the primary or secondary transfer operation can not be well performed. In order to prevent occurrence of such a transfer problem, a transfer bias is applied while controlling the current of the transfer bias (i.e., constant-current controlling). Specifically, when the electric resistance of a transfer roller or an intermediate transfer medium increases, the voltage of the transfer bias applied thereto is increased to prevent decrease of the electric field at the primary or secondary transfer nip, resulting in prevention of occurrence of the transfer problem mentioned above.
However, this technique has a drawback in that when the electric resistance of the transfer roller or intermediate transfer medium seriously increases, the voltage of the transfer bias has to be excessively increased, thereby causing a discharging problem such that discharging (abnormal discharging) occurs or a transfer bias increasing problem such that a transfer bias beyond the capacity of an electric power source has to be applied.
Published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 2003-131498 discloses a technique in that an antioxidant is added to a transfer roller to prevent occurrence of the electric resistance increasing problem. In addition, JP-A 2004-252134 discloses an image forming apparatus using an intermediate transfer belt, which has a surface resistivity property such that when the intermediate transfer belt is subjected to application of a voltage, followed by ground discharging 1000 times while measuring the surface resistivity thereof, the absolute value of the logarithmic difference between the first surface resistivity and 1000th surface resistivity is not greater than 0.5 [log(Ω/□)]. It is described therein that by using an intermediate transfer medium having such a property, occurrence of the electric resistance increasing problem can be prevented.
In order to well perform a primary transfer operation and a secondary transfer operation, the voltage of the transfer bias is determined depending on the volume resistivity of a combination of the transfer roller and the intermediate transfer medium (this volume resistivity is hereinafter sometimes referred to as the combined volume resistivity). The combined volume resistivity changes depending on variables such as resistivity of each of the transfer roller and the intermediate transfer medium, thickness of the elastic layer of the transfer roller, and thickness of the intermediate transfer medium. Therefore, even when the resistivity of a transfer roller (or an intermediate transfer medium) is controlled, the combined volume resistivity changes if the resistivity of the intermediate transfer medium (or the transfer roller) used in combination of the transfer roller (or the intermediate transfer medium) changes. Therefore, it is hard to prevent occurrence of the transfer bias increasing problem in that the voltage of the transfer bias has to be excessively increased.
In order to prevent occurrence of the problem, it may be possible to periodically watch the resistivity of each of a transfer roller and an intermediate transfer medium to determine whether the resistivity falls in a predetermined resistivity range. However, the costs of the image forming apparatus using this technique increase.
In addition, when a combination of a transfer roller and an intermediate transfer medium is evaluated to determine whether the combination can be used even when repeatedly used, a test in which the combination is set in an image forming apparatus and a large amount of copies (on the order of hundreds of thousands) are produced by the image forming apparatus is performed. This evaluation method takes a long time and is expensive.
Because of these reasons, a need exists for an image forming apparatus which can stably produce high quality images for a long period of time without causing the transfer problem, the transfer voltage increasing problem, etc.